This invention relates to a power supply for an arc lamp, and is particularly concerned with a power supply of the general type comprising a power source and an inverter. The arc lamp is typically a metal halide lamp of the type used for cinematographic and television lighting.
An arc lamp of this type is preferably driven by alternating square wave current so as to avoid modulation of the light output at the supply frequency as would occur if the current supply were sinusoidal. Attempts have been made to achieve a satisfactory square wave output for this purpose.
Canadian Patent No. 1185649 dated Apr. 16, 1985, in the name Lee Electric (Lighting) Limited, discloses a power supply for arc lamps which represents a notable advance over the prior art. The power supply is designed to deliver alternating square wave current for driving an arc lamp, but is more compact and more convenient to use than the power supplies previously used. The power supply essentially comprises a power source feeding an inverter from which the output current is derived, the power source being a constant current source comprising a rectifier, a capacitor, an inductor and a field effect transistor connected in series. The inverter is connected across the capacitor so that current supplied to the inverter is drawn through the field effect transistor. The constant current source is maintained by means of a chopper oscillator for controlling the conduction of the field effect transistor in response to the current drawn by the field effect transistor. The inverter comprises a bridge network of field effect transistors driven by a bridge oscillator to deliver the current output at the required output frequency.
While the power supply disclosed in the above Canadian patent is an improvement over the earlier power supplies, it also has shortcomings. One shortcoming is that the constant current source and the inverter are necessarily controlled by separate oscillators and in consequence the system cannot be truly synchronous. While the lack of synchronism in the system does not necessarily affect the light output adversely, it gives rise to noisy operation. Another shortcoming is that, since the power source is controlled by the chopper oscillator to give a constant current output it cannot provide a constant power output since the voltage is subject to variation. This is a disadvantage because variations in the power level give rise to variations in the quality and spectral distribution of the light output.